Vertical Stacking of Carbon Nanotube Arrays for Current Enhancement and Control

ABSTRACT

Transistor devices having vertically stacked carbon nanotube channels and techniques for the fabrication thereof are provided. In one aspect, a transistor device is provided. The transistor device includes a substrate; a bottom gate embedded in the substrate with a top surface of the bottom gate being substantially coplanar with a surface of the substrate; a stack of device layers on the substrate over the bottom gate, wherein each of the device layers in the stack includes a first dielectric, a carbon nanotube channel on the first dielectric, a second dielectric on the carbon nanotube channel and a top gate on the second dielectric; and source and drain contacts that interconnect the carbon nanotube channels in parallel. A method of fabricating a transistor device is also provided.

FIELD OF THE INVENTION

The present invention relates to transistor devices, and moreparticularly, to transistor devices having vertically stacked carbonnanotube channels and techniques for the fabrication thereof.

BACKGROUND OF THE INVENTION

Semiconducting carbon nanotubes can conduct exceptionally high currentsfor their nanoscale diameter (i.e., diameters of from about 1 nanometer(nm) to about 2 nm). However, in spite of carbon nanotubes' high currentdensity (up to 10⁹ Amps per square centimeter (A/cm²)), their small sizestill limits carbon nanotubes to carrying tens of microamps each.

In the context of using carbon nanotubes to replace silicon as an activechannel material for next-generation transistors, the achievable currentfrom a carbon nanotubes-based device must be comparable to silicontechnology (about 1 milliamp per micrometer (mA/μm)). Difficulty incontrollably fabricating closely packed, parallel (aligned) carbonnanotube arrays has kept the achievable current around 0.02 mA/μm. See,for example, K. Ryu, et al., “CMOS-Analogous Wafer-ScaleNanotube-on-Insulator Approach for Submicrometer Devices and IntegratedCircuits Using Aligned Nanotubes,” Nano Lett., vol. 9, pp. 189-197(2009).

Reports suggest that in order to obtain 1 mA/μm from aligned carbonnanotubes, the pitch from nanotube to nanotube would have to be near 10nm, which is an order of magnitude smaller than the best reproduciblyobtained density to date. See, for example, A. Raychowdhury, et al.,“Variation Tolerance in a Multichannel Carbon-Nanotube Transistor forHigh-Speed Digital Circuits,” IEEE TED, vol. 56, pp. 383-392 (2009).Another alternative is the use of random carbon nanotube networks knownas thin-film transistors, but the current in these systems becomes evenmore limited by percolation and/or poor carbon nanotube-metal contactsfrom bundling. See, for example, Q. Cao, et al., “Medium-Scale CarbonNanotube Thin-Film Integrated Circuits on Flexible Plastic Substrates,”Nature, vol. 454, pp. 495-502 (2008).

Therefore, techniques for increasing the current carrying capacity ofcarbon nanotube-based devices would be desirable.

SUMMARY OF THE INVENTION

The present invention provides transistor devices having verticallystacked carbon nanotube channels and techniques for the fabricationthereof. In one aspect of the invention, a transistor device isprovided. The transistor device includes a substrate; a bottom gateembedded in the substrate with a top surface of the bottom gate beingsubstantially coplanar with a surface of the substrate; a stack ofdevice layers on the substrate over the bottom gate, wherein each of thedevice layers in the stack includes a first dielectric, a carbonnanotube channel on the first dielectric, a second dielectric on thecarbon nanotube channel and a top gate on the second dielectric; andsource and drain contacts that interconnect the carbon nanotube channelsin parallel.

In another aspect of the invention, a method of fabricating a transistordevice is provided. The method includes the following steps. A substrateis provided. A bottom gate is formed embedded in the substrate with atop surface of the bottom gate being substantially coplanar with asurface of the substrate. A stack of device layers is formed on thesubstrate over the bottom gate. Each of the device layers in the stackincludes a first dielectric, a carbon nanotube channel on the firstdielectric, a second dielectric on the carbon nanotube channel and a topgate on the second dielectric. Source and drain contacts are formed thatinterconnect the carbon nanotube channels in parallel.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a substrate on which atransistor device will be fabricated according to an embodiment of thepresent invention;

FIG. 2 is a cross-sectional diagram illustrating a bottom gate havingbeen formed in a trench patterned in the substrate according to anembodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a first dielectrichaving been deposited on the substrate over the bottom gate according toan embodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating a carbon nanotubechannel having been formed on the first dielectric over the bottom gateaccording to an embodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating a second dielectrichaving been deposited over the carbon nanotube channel according to anembodiment of the present invention;

FIG. 6 is a cross-sectional diagram illustrating a top gate having beenformed on the second dielectric according to an embodiment of thepresent invention;

FIG. 7 is a cross-sectional diagram illustrating a second device layerhaving been formed on the first device layer to form a stack of devicelayers according to an embodiment of the present invention;

FIG. 8 is a cross-sectional diagram illustrating a third device layerhaving been formed on the stack according to an embodiment of thepresent invention;

FIG. 9 is a cross-sectional diagram illustrating source and draincontacts having been formed on opposite sides of the stack according toan embodiment of the present invention; and

FIG. 10 is a cross-sectional diagram illustrating an exemplaryconfiguration for interconnecting the gates in the stack according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for obtaining a desired current in carbonnanotube transistors by vertically stacking carbon nanotube arrays.FIGS. 1-10, for example are diagrams illustrating an exemplarymethodology for fabricating a transistor device having verticallystacked carbon nanotube arrays. In general, a transistor includes asource and a drain connected by a channel(s), and a gate which regulateselectron flow through the channel(s). As will be described in detailbelow, with the present techniques, a vertical stacking configurationfor the carbon nanotube arrays (which serve as channels of the device)is employed, with the number of carbon nanotube arrays in the stackbeing tailored to provide the desired current.

Specifically, FIG. 1 is a cross-sectional diagram illustrating asubstrate 102 on which the device will be fabricated. Suitablesubstrates include, but are not limited to, a silicon substrate coveredwith a layer of silicon dioxide (into which the bottom gate will beformed, as described below) and quartz. As shown in FIG. 1, a trench 104has been formed (patterned) in substrate 102. Trench 104 marks thefootprint and location of a local bottom gate to be formed as describedbelow. According to an exemplary embodiment, trench 104 is formed usingan anisotropic dry etch (e.g., reactive ion etching (RIE)) followed by awet chemical etch to undercut trench 104 through a positive resist(e.g., poly (methyl methacrylate) (PMMA)). The undercutting of trench104 is done to prevent the build-up of gate material that is to bedeposited in the next step, thus providing a smooth surface on which tobuild the device.

FIG. 2 is a cross-sectional diagram illustrating a bottom gate 202having been formed in trench 104. Bottom gate 202 is formed by firstfilling a suitable gate material into trench 104. According to oneexemplary embodiment, the gate material comprises a metal or acombination of metals deposited into trench 104 using, e.g., electronbeam evaporation. Any metal(s) can be used and the particular metal(s)selected may vary for p-channel and n-channel devices to tune thethreshold voltage accordingly. By way of example only, suitable gatemetals include, but are not limited to aluminum (Al), titanium (Ti)and/or palladium (Pd). In one example configuration, titanium is firstdeposited into trench 104 followed by palladium.

Alternatively, according to another exemplary embodiment, the gatematerial comprises poly-silicon. The poly-silicon can be dopedaccordingly to attain the desired work function and conductivity. Thetechniques for poly-silicon gate doping are known to those of skill inthe art and thus are not described further herein.

The result is bottom gate 202 embedded in substrate 102. As shown inFIG. 2, a top surface of bottom gate 202 is flush with a surface ofsubstrate 102. Since the surfaces are flush with one another, the topsurface of bottom gate 202 is thus coplanar with the surface ofsubstrate 102. The coplanar bottom gate and substrate provide a flatsurface on which the device can be built.

It is notable that the dimensions of bottom gate 202 can be configuredto address the specific device needs. By way of example only, a width wof the bottom gate can be varied to tune the channel length.

As highlighted above, in order to be able to use this local bottom gateconfiguration as a foundation on which the device can be built, thebottom gate has to be level or flush (i.e., coplanar) with thesurrounding substrate which serves to keep the carbon nanotube channel(that will be formed over the gate) free of any physical distortions,such as kinks or bends, which can adversely affect carrier transport.See, for example, in FIG. 2 where it is shown that a top surface ofbottom gate 202 is coplanar with the surface of substrate 102.

It is notable that, due to production tolerances, in practice the topsurface of bottom gate 202 might end up being slightly higher orslightly lower than the surface of substrate 102. When the differencebetween the surfaces is less than or equal to about 5 nm the surfacesare considered substantially coplanar according to the presentteachings. Specifically, if the top surface of bottom gate 202 is lessthan or equal to about 5 nanometers (nm) higher than the surface ofsubstrate 102 then the top surface of bottom gate 202 is consideredherein to be substantially coplanar with the surface of substrate 102.Similarly, if the top surface of bottom gate 202 is less than or equalto about 5 nm lower than the surface of substrate 102 then the topsurface of bottom gate 202 is considered herein to be substantiallycoplanar with the surface of substrate 102.

A substantially coplanar bottom gate/substrate surface may be achievedin a number of different ways. One way is to deposit the metal orpoly-silicon gate material so as to overfill the trench and then polishthe deposited gate material, e.g., using chemical-mechanical polishing(CMP), down to the surface of the substrate. The same resist mask, e.g.,PMMA, that was used to etch trench 104 can also be used to fill the gatematerial. Thus another way to achieve a substantially coplanargate/substrate surface is by depositing the gate material through thePMMA resist mask so as to fill (to the top of, but not overfilling) thetrench, and then use lift-off in acetone to remove the PMMA, leaving themetal gate flush with the surrounding substrate (the presence of theresist allows for the gate material to remain only in the trenches,since when the resist is removed the gate material on top of the resistwashes away with it).

A stack of device layers is then formed on the substrate over the bottomgate. As will be described in detail below, each device layer in thestack includes a first dielectric, a carbon nanotube channel and a topgate separated from the carbon nanotube channel by a second dielectric.FIG. 3, for example, is a cross-sectional diagram illustrating adielectric 302 (which will serve as a gate dielectric of the device)having been deposited on substrate 102 over bottom gate 202. Accordingto an exemplary embodiment, dielectric 302 comprises aluminum oxide(Al₂O₃) or hafnium oxide (HfO₂) and is deposited using a chemical vapordeposition (CVD) process such as atomic layer deposition (ALD).

FIG. 4 is a cross-sectional diagram illustrating a carbon nanotubechannel 402 having been formed on dielectric 302 over bottom gate 202.According to an exemplary embodiment, carbon nanotube channel 402comprises a carbon nanotube array (aligned or random) that is depositedonto dielectric 302. Techniques for depositing the carbon nanotubearrays can range from solution-based dispersion to substrate transfer.The aligned growth of carbon nanotubes on, e.g., quartz, substrates andthe subsequent transfer of these carbon nanotubes onto a receivingsubstrate have been well documented and as such, these processes are notdescribed further herein. Such transfer processing methods would workvery well with the instant techniques, where the unwanted portions ofthe carbon nanotubes are simply etched away after the transfer. If thecarbon nanotube array deposition is solution-based, the carbon nanotubeswill not likely be as well-aligned, but could be patterned directly ontothe gate region without the need for subsequent etch removal steps.

The density of the carbon nanotubes in each device layer determines thecurrent capacity for that layer. For example, the carbon nanotubedensity in each device layer can vary from about 5 carbon nanotubes permicrometer (μm) to about 200 carbon nanotubes per μm, depending on thesource of the nanotubes.

FIG. 5 is a cross-sectional diagram illustrating a dielectric 502 havingbeen deposited over carbon nanotube channel 402. According to anexemplary embodiment, dielectric 502 has the same composition and isdeposited in the same manner as dielectric 302, described in conjunctionwith the description of FIG. 3, above.

FIG. 6 is a cross-sectional diagram illustrating a top gate 602 havingbeen formed on dielectric 502. Like with bottom gate 202, as describedabove, top gate 602 can comprise a metal(s), such as Al, Ti and/or Pd,or alternatively top gate 602 can comprise poly-silicon doped to attaina desired work function and conductivity. Details regarding the use of ametal(s) or poly-silicon as a gate material were presented above.Whichever gate material is chosen, that gate material can be depositedon dielectric 502 and then patterned to form top gate 602. According toan exemplary embodiment, in order to ensure that dielectric 502 (andsubsequent dielectrics) and top gate 602 (and subsequent gates) remainlocal to the stack, once dielectric 502 is deposited over carbonnanotube channel 402 a resist mask (not shown), e.g., PMMA, is patternedover dielectric 502 so as to cover the dimensions (the footprint) of thestack. An etch around the resist mask is used to remove excessdielectric material from all but the stack area. Then top gate 602 isdeposited and another, second resist mask (not shown), e.g., PMMA, ispatterned over top gate 602 in the same manner to cover the dimensions(the footprint) of the stack. An etch around the second resist mask isused to remove excess top gate material from all but the stack area.After each round of etching, acetone can be used to remove therespective masks.

At this point in the process, a first device layer is now complete. Ashighlighted above, each device layer includes a first dielectric (e.g.,dielectric 302), a carbon nanotube channel (e.g., carbon nanotubechannel 402) and a top gate (e.g., top gate 602) separated from thecarbon nanotube channel by a second dielectric (e.g., dielectric 502).One or more additional device layers will now be fabricated on top ofthe first device layer, in a stack. The purpose of this configuration isto achieve a desired current in the device. In general, the greater thenumber of device layers in the stack the greater the current carryingcapacity of the device, due to the increased number of carbon nanotubechannels present. As will be described in detail below, the multiplecarbon nanotube channels are connected, in parallel, to source and drainregions of the device.

FIG. 7 is a cross-sectional diagram illustrating a second device layerhaving been formed on the first device layer, so as to form a stack ofdevice layers. Like the first device layer, the second device layerincludes a first dielectric (e.g., dielectric 302 a), a carbon nanotubechannel (e.g., carbon nanotube channel 402 a) and a top gate (e.g., topgate 602 a) separated from the carbon nanotube channel by a seconddielectric (e.g., dielectric 502 a). As described above, resist masks(not shown), e.g., PMMA, can be used in conjunction with the formationof the dielectrics (e.g., dielectrics 302 a and 502 a) and the top gate(e.g., top gate 602 a) to ensure that the dielectrics and top gateremain local to the stack. It is notable that in the first device layer,the dielectric over the bottom gate (e.g., dielectric 302) covers thewhole substrate, whereas in subsequent device layers, such as here, thefirst dielectric (e.g., dielectric 302 a) is only in the stack. Thisconfiguration can be achieved through use of an additional lithographyand etching step, e.g., through a PMMA resist mask, during the formationof the second and subsequent device layers, as described above.

As shown in the figures, the carbon nanotube channels do not need to belocal to the stack. In fact, it is advantageous for the carbon nanotubechannels to protrude/extend from the stack so that the source and draincontacts (formed as described below) can envelope those carbon nanotubechannel extensions.

It is notable that with this configuration the top gate of a given oneof the device layers in the stack also serves as a bottom gate for thedevice layer immediately above that given layer (i.e., the next highestlayer) in the stack. Thus, by way of example only, top gate 602 (of thefirst device layer) also serves as a bottom gate for the second devicelayer. With the present device configuration, all of the bottom and topgates are interconnected to create a single gate (i.e., one common gate)of the device that modulates the carbon nanotube channel from the bottomand top, respectively, thus improving the electrostatics.

FIG. 8 is a cross-sectional diagram illustrating a third device layerhaving been formed on the stack, i.e., on the second device layer, bythe same process and with the same materials used to form the seconddevice layer. Like the first and second device layers, the third devicelayer includes a first dielectric (e.g., dielectric 302 b), a carbonnanotube channel (e.g., carbon nanotube channel 402 b) and a top gate(e.g., top gate 602 b) separated from the carbon nanotube channel by asecond dielectric (e.g., dielectric 502 b). Additional device layers(i.e., fourth, fifth device layers, etc.) can be added to the stack inthe same manner (i.e., the number of device layers formed can be varied)to tune the characteristics of the device. The number of layers isdetermined by the density of carbon nanotubes that are being used ineach device layer which, as described above, determines the current foreach device layer. Thus, for device layers of a given carbon nanotubedensity, increasing the number of device layers increases the overallcurrent capacity of the device. For example, if each device layer has adensity of 50 carbon nanotubes for the designated device width w (seeFIG. 2, described above), and each nanotube carries 20 microamps (μA) ofcurrent, then each device layer will provide 1 milliamps (mA) ofcurrent. Therefore, for a 3 mA drive current device, three device layerswill be needed.

As highlighted above, the present techniques address the challenge ofobtaining enough carbon nanotubes in a transistor channel to realizeon-currents comparable to those in silicon complementary metal-oxidesemiconductor (CMOS). A concept utilized herein is that the carbonnanotube channel material does not have to be substrate-bound, thusallowing for the present vertical integration (stacking) approach. Oneof the foremost advantages of this approach is that each layer ofchannel material (carbon nanotube array) is kept isolated from theunder/overlying parallel layers, eliminating 1) any charge screeningeffects between layers that could hamper performance and 2) potentialcarbon nanotube bundling that can occur when depositing carbon nanotubearrays on top of one another. The vertical integration also provideseach carbon nanotube in the transistor with a top and bottom gate, thusimproving the electrostatics. Another advantage is that this verticalintegration allows for smaller (scalable) footprint devices.Furthermore, this method is highly customizable (i.e., tunable),including the ability to design devices that incorporate a certainnumber of vertically stacked carbon nanotube layers to achieve desiredon-currents, and the ability to stack enough layers to achieve currentswell beyond silicon CMOS (see above). The present techniques can beimplemented to produce transistor devices that are highly customized tosuit a particular layout or process technology.

Once the appropriate number of device layers have been fabricated,source and drain contacts are formed. FIG. 9 is a cross-sectionaldiagram illustrating source and drain contacts 902 and 904 having beenformed on opposite sides of the stack. As highlighted above, the carbonnanotube channels preferably protrude/extend from the stack. As shown inFIG. 9, source and drain contacts 902 and 904 envelope those carbonnanotube channel extensions. Accordingly, source and drain contacts 902and 904 interconnect the device layers in parallel. Specifically, sourceand drain contacts 902 and 904 interconnect the carbon nanotube channelsof the various device layers in parallel. The source and drain contactsmay be formed by depositing metal through a resist mask and then liftingoff. A variety of other suitable methods known to those of skill in theart may be similarly employed to form the source and drain contacts.

As described above, with the present device configuration, all of thebottom and top gates are interconnected to create a single gate (i.e.,one common gate) of the device that modulates the carbon nanotubechannel from the bottom and top, respectively, thus improving theelectrostatics. According to an exemplary embodiment, the bottom and topgates are interconnected away from the active device. See, for example,FIG. 10.

FIG. 10 is a cross-sectional diagram illustrating an exemplaryconfiguration for interconnecting the top and bottom gates in the stack.The depiction shown in FIG. 10 is representative of a cross-sectionalcut through the device layer stack and substrate, e.g., of FIG. 9, alongline A1-A2 which is perpendicular to the cross-sectional view shown inFIG. 9. As shown in FIG. 10, all of the gates 202, 602, 602 a and 602 bare joined (interconnected) at block 1002. According to an exemplaryembodiment, block 1002 is simply a patterned block of gate material,i.e., the same gate material (metal or doped poly-silicon) as the gates.Block 1002 may be fabricated by depositing the gate material through aresist mask and then lifting off.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

1. A transistor device, comprising: a substrate; a bottom gate embeddedin the substrate with a top surface of the bottom gate beingsubstantially coplanar with a surface of the substrate; a stack ofdevice layers on the substrate over the bottom gate, wherein each of thedevice layers in the stack includes a first dielectric, a carbonnanotube channel on the first dielectric, a second dielectric on thecarbon nanotube channel and a top gate on the second dielectric; andsource and drain contacts that interconnect the carbon nanotube channelsin parallel.
 2. The device of claim 1, wherein the substrate comprises asilicon substrate covered with a layer of silicon dioxide, and whereinthe bottom gate is embedded in the silicon dioxide layer.
 3. The deviceof claim 1, wherein the top gate and the bottom gate each comprise oneor more metals.
 4. The device of claim 1, wherein the top gate and thebottom gate each comprise doped poly-silicon.
 5. The device of claim 1,wherein the top gate of a given one of the device layers also serves asa bottom gate for a next highest one of the device layers in the stack.6. The device of claim 1, wherein the first dielectric and the seconddielectric each comprise one or more of aluminum oxide and hafniumoxide.
 7. The device of claim 1, wherein the carbon nanotube channelcomprises a carbon nanotube array.
 8. The device of claim 7, wherein thecarbon nanotube array is aligned.
 9. The device of claim 7, wherein thecarbon nanotube array is random.
 10. The device of claim 1, wherein anumber of the device layers in the stack can vary according to a desiredcurrent carrying capacity of the device.
 11. A method of fabricating atransistor device, comprising the steps of: providing a substrate;forming a bottom gate embedded in the substrate with a top surface ofthe bottom gate being substantially coplanar with a surface of thesubstrate; forming a stack of device layers on the substrate over thebottom gate, wherein each of the device layers in the stack includes afirst dielectric, a carbon nanotube channel on the first dielectric, asecond dielectric on the carbon nanotube channel and a top gate on thesecond dielectric; and forming source and drain contacts thatinterconnect the carbon nanotube channels in parallel.
 12. The method ofclaim 11, wherein the step of forming the bottom gate embedded in thesubstrate comprises the steps of: forming a trench in the substrate;filling the trench with a gate material; and polishing the gate materialdown to the surface of the substrate.
 13. The method of claim 12,further comprising the step of: undercutting the trench.
 14. The methodof claim 13, wherein the trench is undercut using a wet chemical etch.15. The method of claim 13, wherein the step of forming the stack ofdevice layers comprises the steps of, for each device layer: depositingthe first dielectric on a preceding device layer in the stack; formingthe carbon nanotube channel on the first dielectric; depositing thesecond dielectric on the carbon nanotube channel; and forming the topgate on the second dielectric.
 16. The method of claim 11, wherein thecarbon nanotube channel comprises a carbon nanotube array.
 17. Themethod of claim 16, wherein the carbon nanotube array is aligned. 18.The method of claim 16, wherein the carbon nanotube array is random. 19.The method of claim 11, further comprising the step of: varying a numberof the device layers in the stack to achieve a desired current carryingcapacity for the device.